Nephronophthisis (NPHP) is the most common genetic cause of end stage renal disease in infants, children, and young adults. NPHP is caused by a mutation in one of at least nine different genes (NPHP1 - NPHP9), accounting for less than 50% NPHP cases and indicating that many other disease loci remain unidentified. NPHP and other cystic kidney diseases are associated with defects in cilia. While the NPHP gene products (the nephrocystins) are localized to cilia, their functions in this sensory organelle remain largely unknown. The nematode Caenorhabditis elegans is a powerful model organism to study the roles of the nephrocystins in their native cellular environment. In C. elegans, NPHP-1 and NPHP-4 act globally to modulate ciliary development and morphogenesis in a cell-type specific manner. Human and worm nephrocystin-1 and nephrocystin-1 localize to the transition zone of cilia on renal epithelial cells and sensory neurons, respectively, suggesting an evolutionarily conserved role. Proposed studies in Aim 1 will define how NPHP-1 and NPHP-4 function at the ciliary transition zone. Proposed studies in Aim 2 will determine the role of the C. elegans NPHP2, NPHP8, and NPHP9 homologs. Proposed studies in Aim 3 will reveal genetic and functional interactions between the NPHP genes and known ciliopathy disease gene homologs. An understanding of human ciliary diseases such as Nephronophthisis relies on a complete understanding of ciliary components and of complex genetic and developmental interactions with modifier loci. These proposed studies will broaden our understanding of the nephrocystins and cilia biology at the genetic, molecular, cellular, and organismal levels. Such understanding is essential in order to identify the functions of the NPHP genes, their role in disease processes, and their potential as therapeutic targets.